The solid-oxide fuel cell (SOFC) generates clean energy by converting hydrogen into electricity. In addition, it can store some electrochemical power that can be used once the hydrogen has run out.

The demonstration, outlined in the latest Nano Letters journal, was carried out by materials scientists from America's Harvard University.

Main investigator Shriram Ramanathan, who is also Associate Professor of Materials Science at Harvard School of Engineering and Applied Sciences (SEAS) says, "This thin-film SOFC takes advantage of recent advances in low-temperature operation to incorporate a new and more versatile material. Vanadium oxide (VOx) at the anode behaves as a multifunctional material, allowing the fuel cell to both generate and store energy."

Left: Each dark speck within the nine white circles at left is a tiny fuel cell. An AA battery is shown for size comparison. (Photo by Caroline Perry, SEAS Communications.) Right: One of the nine circles is magnified in this image, showing the wrinkled surface of the electrochemical membrane. (Micrograph by Quentin Van Overmeere.)

Smaller, portable appliances that require light power that can be interrupted, such as unmanned aerial vehicles, could benefit most from the technique, says main author, Quentin Van Overmeere, a postdoctoral fellow at SEAS.

If it is not possible to carry out a refuelling in-situ, using the stored power could increase the appliance's lifespan, he adds.

The team, which includes co-author, SEAS graduate student, Kian Kerman, usually work on thin-film SOFCs with platinum electrodes, but when the anode power runs out, the SOFC can generate just 15 seconds of extra power from the electrochemical reaction.

The latest SOFC has a double layer of platinum and vanadium oxide for the anode that means the cell can keep going without fuel for up to 14 times longer - 3 minutes, 30 seconds - at 0.2 mA/cm2 current density.

Three reactions potentially occur within the new SOFC. The vanadium ions oxodise, hydrogen is stored in the VOx crystal lattice and is released over time and oxodised at the anode and the amount of oxygen ions may differ between the anode and cathode, so oxygen anions may be oxidized as happens in a concentration cell.

Three possible mechanisms (left to right) can explain the operation of the vanadium oxide / platinum fuel cell after its fuel has been spent. The illustration represents a simplified cross-section of the SOFC: the top layer is the cathode (made of porous platinum), the middle layer is the electrolyte (yttria-stabilized zirconia, YSZ), and the bottom layer is the VOx anode. During normal operation, the hydrogen fuel would be at the bottom of this diagram. (Image courtesy of Quentin Van Overmeere.)

All three reactions can feed electrons into a circuit, but it is not known what allows the new fuel cell to keep running. Tests by the team suggest at least two of three mechanisms are simultaneously in play.

But it is early days in developing the reaction and further improvements could be made to the way the VOx-platinum anode is made that could increase the lifespan of the cell. The team believes it could take a couple of years to produce a more advanced SOFC that is ready for testing in applications.

When operating normally, it produces a similar amount of power to a platinum-anode SOFC. Meanwhile, the special nanostructured VOx layer sets up various chemical reactions that continue after the hydrogen fuel is spent.

Purple plasma is visible through the window of this vacuum deposition chamber. The equipment is used for creating the extremely thin-layered electrodes and electrolyte on a wafer of silicon. (Photo Credit: Caroline Perry, SEAS Communications.)

The research was backed by the U.S. National Science Foundation, a postdoctoral scholarship from Le Fonds de la Recherche Scientifique and the U.S. Department of Defense's National Defense Science and Engineering Graduate Fellowship Program.

The team also received resources from Harvard University Center for Nanoscale Systems and MRSEC Shared Experimental Facilities at MIT, the Massachusetts Institute of Technology.

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